1. Field of the Invention
The present invention relates to a fixing device used for heating and pressurizing an unfixed image to be fixed in an image forming apparatus such as a copier, a printer, or a facsimile, and particularly, to a sliding member (a sheet-like member) for an electrophotographic apparatus interposed between a press member which presses a tubular body for fixing, from inside of the tubular body toward a fixing member side, in order to form a nip portion through which a recording medium passes, and the tubular body; and a fixing device using the sliding member.
2. Description of the Related Art
In forming an electrophotographic image by a printer, a copier, a facsimile or the like, it is necessary for a recording sheet or the like on which an unfixed toner image is formed, to be heated and pressurized for fixing the toner image by passing through an image fixing device. A belt nip scheme has been publicly known in which a film tubular body made of heat resistant plastic is used as the image fixing device. In the belt nip scheme, the film tubular body is circumscribed about a driving fixing roll, an elastic press member is inscribed inside of the film tubular body in the circumscribed portion, a sliding sheet is provided between them and oil is applied on the sliding sheet, whereby a nip portion is formed between the fixing roll and the film tubular body and the toner image is fixed on the recording sheet while the recording sheet passes through the nip portion.
In the nip belt scheme, it is inevitable to prevent slippage between the fixing roll and the recording sheet, and slippage between the recording sheet and the film tubular body in order to ensure an excellent fixed image and fixing property. Hence, if a friction coefficient between the fixing roll and the recording sheet is denoted by μa, a friction coefficient between the recording sheet and the film tubular body is denoted by μb and a friction coefficient between the film tubular body and the elastic press member is denoted by μc, it is at least required to satisfy both of the relations, μa>μc and μb>μc. Conventionally, in order to reduce the friction coefficient μc, there have been made proposals in which the elastic press member is covered with a covering layer (a low friction sheet) made of a glass fiber sheet which has been coated with fluorocarbon resin and baked, and one of various kinds of modified silicone oils as a lubricant is placed between the covering layer and the film tubular body (for example, Japanese Patent Application Laid-Open (JP-A) Nos. 10-213984 and 2001-249558).
Such a conventionally used covering layer (low friction sheet) made of a glass fiber sheet which has been coated with fluorocarbon resin and baked, has a porous surface at least on the face in sliding contact to the inner surface of the film tubular body, in order to retain a lubricant thereon. However, it has been found that since the sliding surface of the low friction sheet is porous, problems still remain due to insufficiency with respect to the following aspects.
That is, in the usage over a long term, a fluorocarbon resin layer coated on the outermost surface of the covering layer is worn out to expose a glass fiber sheet serving as a reinforcing substrate. And the inner surface of the belt is worn out by the contact with the glass fiber, which deteriorates reliability of the belt. Further, worn-out powders are accumulated, and there arises an occasion in which and the glass fiber surface directly contacts with the inner surface of the belt to increase a friction coefficient between the inner surface of the film tubular body (an endless belt) and a surface of the low friction sheet. It in turn increase a driving torque for the fixing roll. As a result, a stress increases that acts on a gear receiving section with a small thickness of a fixing roll core, causing breakage of the gear and the core. Furthermore, as a matter of course, it increases a load on a motor.